This invention relates to x-ray tubes in general and more particularly to an improved disc-shaped composite anode for use as a rotary anode in an x-ray tube.
Rotary anodes for x-ray tubes which consist of a layered, pressed and sintered anode body comprising a support body of a tungsten-molybdenum alloy with a first layer of tungsten or tungsten alloy on the surface which is impinged electrons, referred to as a spot track, and having on its opposite surface a second layer of a high melting point material are known.
For example, U.S. Pat. 2,863,083 teaches a rotary anode for x-ray tubes fabricated by the electro-deposition of several layers. The anode comprises a support body of molybdenum partially covered with the intermediate layer of tungsten. On the intermediate layer an x-ray active layer of rhenium is placed. With a design of this nature the cooling down speed of the anode is supposedly increased considerably.
In addition, a disc shaped composite anode for use as a rotary anode for x-ray tubes and comrising a layered, pressed, and sintered anode body of a high melting point material in which at least the surface layer which is impinged by electrons is an x-ray active layer of a tungsten alloy is known. To prevent formation of cracks in the anode body even with high loads two further layers are provided beneath the x-ray active first layer. The second layer consists of pure tungsten of tungsten alloy with a high tungsten content of at least 70% tungsten and a third layer beneath that consists of a molybdenum alloy.
In practice, it has been discovered that an anode disc of this nature warps even after a short period of operational use. Specifically, the anode angles which were originally 10.degree. to 20.degree. changed to 8.degree. or less. As a result image quality is degraded and in extreme cases the rotary anode becomes useless.
Tests have shown that with a cold start i.e. where an anode cooled down to room temperature is suddenly loaded, as well as with a hot start where the anode is already at a uniform background temperature of e.g. 700.degree., loading times of 5-10 seconds or 2-20 seconds respectively normally produce stresses at a depth of 1 mm from the surface of the rotary anode, for example, which exceed the elastic limit of the material. In the case of a hot start such conditions can prevail to a depth of even 5-6 mm. This corresponds to approximately the entire thickness of the rotary anode.
This warping of the anode discs causing their angles to be decreased can be understood from the fact that in operation the thermal expansion of the relatively heat resistant x-ray active layer of a tungsten alloy, for example, forces elongation of the support body, which may consist of a tungsten-molybdenum alloy, beyond its elastic limit because of the temperature distribution. This plastic deformation at temperatures above 1400.degree. C is not reversable during cooling down. As a result the angle becomes smaller with increasing operating time. The process takes place in small steps but eventually leads to intolerably large angle changes and in an extreme case continues until the anode becomes flat.
Various measures are known for increasing the stability of the support body. For example it is known that it can be done by increasing the cross-section or through the use of ribs. However, these methods provide only a slight improvement of the angular stability. Furthermore, such can be obtained only if the cross sections are substantially increased or the ribs brought out of the solid body for a relatively large distance. Either of these solutions is accompanied by a considerable increase in weight and, in the case of a ribbed design, by more wear on drop forge dies used in making the anodes. This leads to an undesirable increase in the cost of the rotary anode. Furthermore, a more powerful drive and more expensive bearings for the rotary anode are required.
In view of these problems with prior art anodes, the need for an improved rotary x-ray anode in which no warping takes place even under high loads becomes evident.